What I've done is set :

$sa_tag_level_deflt  = -999.0
$sa_tag2_level_deflt = 4.9
$sa_kill_level_deflt = 20.0
$final_spam_destiny = D_DISCARD

• So all messages get the spam headers
• All mail scoring >4.9 is flagged as spam but passed on to intended recipient
• All mail scoring >20 is discarded